Analog-to-digital conversion is widely used in electronic systems. Some conventional electronic systems package an analog-to-digital converters (ADC) on a separate integrated circuit. To reduce the size, power consumption, and manufacturing and design cost, it is often desirable to integrate ADCs and other functional circuits on a single circuit chip. Such integration also improves the system reliability, noise resistance and chip-to-chip interfacing.
One application for the integrated ADC technology is in the field of semiconductor imaging devices. A typical semiconductor imager includes an array of photo sensors and processing circuits. An optical image captured by the imager is first converted into analog electrical signals and then processed by the processing circuits in either analog mode or digital mode. A digital image sensor senses an optical image and outputs a digital representation of the image. Many applications requires digital image sensors for digital image manipulation, digital data storage and transmission. Therefore, analog-to-digital converters are needed in the signal train of a digital image sensor.
In addition to considerations of circuit size, power consumption, and cost, integrating ADCs with a sensing array of a digital image sensor allows the analog signals to be converted into the digital format at an early stage in the signal chain and thus significantly improves the noise performance. This is in part because an early analog-to-digital conversion reduces signal degradation due to noise and nonlinearities in analog transmission and processing.